The present invention relates generally to third generation wireless communications, and in particular, the present invention relates to synchronization of a mobile station during handover between a third generation communication system and the Global System for Mobile Communications system.
Problems associated with the interoperability of third generation universal mobile telephone service (UMTS) with existing services, such as the Global System for Mobile Communications (GSM), have been identified. In particular, one of the primary concerns in the standardization of third generation services, such as UMTS, is the definition of methods which permit inter-service cell reselection and handover from GSM to UMTS, and visa-verse.
During the initial deployment, it is important that UMTS systems interoperate with existing GSM systems, since, for a given effective radiated power level, a GSM transmission at 900 MHz exhibits a much better ability to penetrate most modern building materials, when compared to a UMTS transmission at 2100 MHz. As a result, during the first stages of UMTS deployment in an existing GSM environment, multiple handovers between GSM and UMTS will be likely, as mobile users move deep inside buildings where GSM at 900 MHz would likely provide a more reliable path than UMTS at 2100 MHz. A clearly specified set of inter-working functions to address these type of operational issues will therefore be necessary, at least during initial deployment of third generation UMTS while microcells are being deployed to fill in coverage gaps in the new UMTS environment.
In order to prepare for a handover from GSM to UMTS, the mobile station must acquire synchronization on the UMTS target cell prior to the handover while simultaneously involved in dedicated traffic flow on a GSM traffic channel (TCH). However, operations associated with acquiring synchronization to a UMTS cell are unacceptably long and will exceed the amount of time normally available (i.e. schedulable) during a GSM circuit-switched voice call, causing the handover to be problematic.
For example, in order to acquire synchronization on UMTS, a typical period of 10 to 12 free GSM idle frames would be required under ideal conditions. Assuming that all idle frames are free, then the actual period required would be 1.44 seconds. In reality, the time required for synchronization to UMTS is heavily influenced by the variable quantity expressed as the ratio of the energy per chip to base channel interference level (E c /I O), and may range from 1.44 seconds, under ideal E c /I O conditions, to between 5 and 16 seconds, in the case of fast-moving mobile stations and poor E c /I O. This dependency is further complicated by the fact that a GSM idle frame must actually be free, and therefore available for use by the UMTS synchronization procedure. The actual time required in a real GSM/UMTS operational environment is likely to be greater than the estimates of minimum 1.44 seconds to a maximum of 16 seconds.
In addition, a significant portion of the synchronization period may be accounted for by the assumed method""s dependency on idle frames, i.e., that the actual idle periods of time in which synchronization activities may be performed are relatively short (4.6 ms), and that idle periods themselves are relatively infrequent, having a periodicity of 120 ms.
Accordingly, what is needed is a method for synchronizing a mobile station during handovers between GSM and UMTS services.